Removing aluminum nitride sections

ABSTRACT

Approaches for substantially removing bulk aluminum nitride (AlN) from one or more layers epitaxially grown on the bulk AlN are discussed. The bulk AlN is exposed to an etchant during an etching process. During the etching process, the thickness of the bulk AlN can be measured and used to control etching.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with Government support under U.S. ArmyCooperative Agreement No. W911NF-10-02-0102 awarded by the DefenseAdvanced Research Projects Agency (DARPA). The Government has certainrights in this invention.

SUMMARY

Approaches for substantially removing or thinning aluminum nitridesections involve complete removal or chemically thinning to a thicknessless than about 50 μm. In some embodiments, an apparatus includes achamber configured to expose a bulk aluminum nitride (AlN) substrate toan etchant during an etching process. The bulk AlN substrate is asubstrate of a subassembly that includes one or more epitaxial layersgrown on the bulk AlN substrate. The apparatus also includes ameasurement unit configured to generate a signal indicative of athickness of the bulk AlN substrate.

Some embodiments involve a method of substantially removing a bulkaluminum nitride (AlN) substrate from a subassembly that includes one ormore epitaxial layers grown on the AlN substrate. The AlN substrate isexposed to an etchant during an etching process. The thickness of theAlN substrate is measured and the etching process may optionally becontrolled based on the measured thickness of the AlN substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that depicts a subassembly that initiallyincludes a bulk AlN substrate and epitaxial layers formed on a bulkaluminum nitride (AlN) substrate;

FIG. 2 illustrates a method of removing a bulk AlN substrate from asubassembly in accordance with some embodiments;

FIG. 3 is a flow diagram that illustrates measuring the thickness of anAlN substrate using light that is transmitted through the subassembly inaccordance with some embodiments;

FIG. 4 graphically illustrates etch times of epitaxial Al_(x)Ga_((1-x))Nlayers in phosphoric acid;

FIG. 5 is a graph of the etch rate of Al_(x)Ga_((1-x))N layers inphosphoric acid;

FIG. 6 is an example of the x-ray diffraction measurement of a samplesubassembly before and after removal of the substrate using methodsdisclosed herein;

FIG. 7 is a block diagram of an apparatus that can be used to remove anAlN substrate from a subassembly by wet etching in accordance with someembodiments;

FIG. 8 illustrates the etch rate of bulk AlN as a function oftemperature, the bulk AlN etching accomplished using methods disclosedherein;

FIG. 9 shows superimposed graphs of profilometer measurements of thesubassembly;

FIG. 10 is a block diagram of an apparatus that uses light to generate asignal indicative of the thickness of the AlN substrate in accordancewith some embodiments;

FIG. 11 illustrates an apparatus that uses light to generate a signalindicative of the thickness of the AlN substrate in accordance with someembodiments;

FIG. 12 is a block diagram of an apparatus that uses light generated bythe subassembly to generate a signal indicative of the thickness of theAlN substrate in accordance with some embodiments;

FIG. 13 depicts an example of a signal generated by the photodetector asa function of the thickness of the AlN layer;

FIG. 14 shows a ultraviolet (UV) light emitting diode (LED) that isinitially grown on a bulk AlN substrate which is subsequently removed inaccordance with embodiments disclosed herein;

FIG. 15 shows a ultraviolet light emitting diode that includes an etchreduction layer in accordance with some embodiments;

FIG. 16 illustrates a vertical electrical injection LED architecturehaving back side contacts at the surface where the substrate was removedin accordance with some embodiments;

FIG. 17 depicts an energy barrier diagram of laser diode design havingan AlN substrate removed according to the processes discussed herein;

FIGS. 18 and 19, respectively, illustrate the resulting epitaxiallygrown template/substrate and the process of forming a stand-aloneepitaxially grown template/substrate in accordance with someembodiments; and

FIG. 20 shows the resulting structure following separation of anepitaxially grown heterostructure from a substrate by etching aninterface layer in accordance with some embodiments; and

FIG. 21 is a flow diagram that depicts the process of forming anepitaxially grown heterostructure by etching an interface layer toseparate the heterostructure from the substrate in accordance with someembodiments.

DESCRIPTION OF VARIOUS EMBODIMENTS

Light emitting devices operating in the 200 to 365 nm range canpotentially be used in a number of applications, including waterpurification, disinfection, UV curing, phototherapy, and medicaldiagnostics. Devices operating in this wavelength range have beenfabricated based on group III-nitride system alloys comprising one ormore of aluminum (Al), gallium (Ga), indium (In), boron (B) along withnitrogen (N). Fabrication of these devices can involve epitaxiallygrowing material layers on a bulk crystalline aluminum nitride (AlN)substrate. Unlike devices utilizing other types of substrates, such asGaAs or sapphire substrates, devices grown on bulk AlN substrates havepreviously been inseparable from their substrate because AlN substratesare not successfully removed using the techniques developed for othertypes of substrates.

Chemical processes for etching AlN crystal wafers are described herein.In some implementations, the nitrogen polar face of the bulk AlNsubstrate is selectively etched. These techniques enable a newgeneration of AlN-grown high-bandgap devices that can be separated fromtheir native substrate.

Approaches described this disclosure involve epitaxially growing one ormore layers on bulk crystalline AlN substrates and subsequentlysubstantially removing the bulk AlN substrate. These approaches may beused in the fabrication of various types of devices that incorporatelight emitting layers and/or may be used in the fabrication ofstand-alone epitaxially grown templates/substrates and/or may be usefulto fabricate other type of subassemblies.

In some cases, the epitaxial layers of a subassembly that was grown onan AlN substrate may form an optoelectronic device as a light emittingdiode (LED) or a laser diode (LD) that has its AlN substratesubstantially removed. In some cases, the subassembly may includeepitaxial layers without a light emitting layer. In these cases, afterthe bulk AlN substrate is removed, the epitaxially grown layers areuseful as stand-alone epitaxially grown AlN templates/substrates. Theseepitaxially grown templates/substrates can be sold as a piece partswhich may be incorporated into various device structures in subsequentprocessing steps.

Bulk crystalline AlN substrates are formed by growing a single crystalAlN boule from a seed crystal and slicing the boule into wafersubstrates. Layers of materials, e.g., III-nitride materials that aresubstantially lattice matched to the bulk AlN substrate may beepitaxially grown on the wafer substrates with high crystalline qualityto form the subassemblies discussed herein. The subassembly layers havea surface nearest the bulk AlN substrate which is the initial surface ofepitaxial growth. After removal of the bulk AlN substrate, theepitaxially grown layers can have a low defect density near the initialsurface of epitaxial growth due to the substantial lattice match withthe bulk crystalline AlN substrate. For example, the defect density ofthe epitaxial layers may be lower than about 10⁸ cm⁻³ or in a range ofabout 10¹⁰ cm⁻³ to about 10³ cm⁻³ within about 100 nm of the initialsurface of epitaxial growth. The defect density of the epitaxial layerscan be related to the defect density of the substrate. For example, thedefect density of the epitaxial layers may be in range of about 10 toabout 1000 times the defect density of the substrate.

Substantial removal of the substrate involves removing the substrateover a majority of the initial surface of epitaxial growth. Substantialremoval of the substrate may involve thinning, e.g., by chemicallyetching, the AlN substrate over a majority of the initial surface ofepitaxial growth. In some cases, a substrate remnant may remain at overa majority of the initial surface of epitaxial growth after the etchingprocess. The substrate remnant has a thickness between about 50 μm and 0nm (completely removed) or less than about 50 μm.

Bulk AlN substrates are substantially optically absorbing for somewavelength ranges, e.g., for wavelengths above 365 nm and/or in the UVwavelength range of about 200 nm to about 365 nm, because of colorcenters, impurities, and/or other defects in the material. For lightemitting devices, substantial removal of the bulk AlN substrate allowslight to emerge from the substrate side of the device (denoted herein asthe back side of the device), i.e., through the surface where the bulkAlN was substantially removed. Substantial removal of the bulk AlNsubstrate can also provide access to the backside of the device forother purposes such as surface texturing and/or electrical contacts. Forstand-alone epitaxial templates/substrates, substantial removal of thebulk AlN substrate allows the epitaxial template/substrate to besubstantially optically transparent to light having wavelengths above365 nm and/or in the wavelength range of about 200 nm to about 365 nm.

In some light emitting device designs, it is particularly helpful toextract light from the back side of the device, because layers above theactive light emitting layers are optically absorbing, at least to someextent. In the case of laser diodes, an optically absorbing substratecontributes to waveguide materials loss, thus increasing the requiredgain to achieve lasing. Aluminum nitride substrates are electricallyinsulating, so leaving these substrates on the device precludes verticalinjection laser diode architectures which have back side contacts.

FIG. 1 is a block diagram that depicts a subassembly 100 that initiallyincludes a bulk crystalline AlN substrate 140 and epitaxial layers 150formed on the bulk AlN substrate 140. Although FIG. 1 illustratesseveral epitaxial layers 150, the subassembly may include more or fewerepitaxial layers and in some implementations may include only oneepitaxial layer. The AlN substrate 140 has a first surface 141 and asecond surface 142. The bulk crystalline AlN substrate may be grown asan ingot that is sliced at the first and second surfaces 141, 142.

In some cases, as illustrated in FIG. 1, epitaxial layers 150 includeone or more light emitting layers 120 sandwiched in between first andsecond heterostructures 110, 130. The light emitting layers 120 and/orthe heterostructures 110, 130 may be formed of GaN, AlGaN, InN, InGaN,and/or InAlGaN, as these material systems can be grown epitaxially onthe AlN with low defect density. In this example, the firstheterostructure 110 is epitaxially grown above the bulk AlN substrate140. The growth of the epitaxial layers 150 proceeds from the firstsurface 101 of the first heterostructure 110 which grows epitaxially onthe second surface 142 of the AlN substrate 140. The firstheterostructure 110 may comprise multiple layers including defectreduction layers, strain management layers, and/or electrical currentdistribution layers. The active region 120 is subsequently grown abovethe first heterostructure 110 and the second heterostructure 130 isgrown above the active region 120. As indicated by the dashed line 160,after formation of the subassembly 100, the bulk AlN substrate 140 issubstantially removed.

FIG. 2 illustrates a method of removing a bulk AlN substrate from asubassembly. The subassembly comprises the bulk AlN substrate and one ormore epitaxially grown layers on the bulk AlN substrate. For example,the one or more epitaxial layers may comprise a light emitting devicehaving a light emitting layer or may comprise epitaxial layers that donot include a light emitting layer.

The bulk AlN substrate, e.g., the nitrogen-polar (N-polar) face of thesubstrate, is exposed to an etchant comprising phosphoric acid that isat an elevated temperature, e.g., at the boiling point of the phosphoricacid, in a wet etching process 220. In the LED structures describedherein, e.g., as schematically illustrated in FIG. 1, the bottom of thebulk AlN substrate can present an N-polar face to the etchant.Phosphoric acid in a concentration range equal to or greater than about85% or in a range of about 87% to about 96% and a temperature range of160° C. to 210° C. can dissolve bulk single crystal AlN within areasonable time. In some configurations, the phosphoric acid used forcan have a concentration range of 87% to 92% with a temperature range ofabout 160° C. to about 180° C., for example. This solution can also beused to etch AlN substrates presenting an Al-polar surface after aninitial hydrolysis of the surface.

The etch process parameters, e.g., concentration and/or temperature ofthe phosphoric acid may be monitored 225 during the etching process andmay be controlled so that the concentration and/or temperature of thephosphoric acid remains within predetermined concentration/temperatureranges. During the etching process, the thickness of the AlN substratecan be monitored 230. In some cases, the thickness of the AlN substrateis measured ex situ which involves periodically removing the subassemblyfrom the etching process, measuring the thickness of subassembly and/orthe thickness of the aluminum nitride substrate, and possibly returningthe subassembly to the etching process after the measurement. In othercases, the thickness of the subassembly and/or the AlN substrate can bemeasured in situ as discussed below.

The etching process can be optionally controlled 240 based on themeasured thickness of the subassembly and/or AlN substrate. The etchingprocess can be controlled to change the etch rate (increase or reducethe etch rate), to start the etching process, and/or to stop the etchingprocess after the substrate reaches a predetermined state, e.g. thesubstrate has been substantially removed from the subassembly or whenthe substrate reaches a predetermined thickness, e.g., about 50 mm, orwhen the substrate has been substantially removed in a region of thesubassembly. The etch rate can be controlled by changing theconcentration and/or temperature of the etchant. For example, as thethickness of the AlN approaches a predetermined thickness (or approachessubstantial substrate removal), the etch rate may be slowed by addingwater to the chamber to decrease concentration of the etchant.Alternatively or additionally, the temperature of the etchant may bereduced to slow the etch rate. In some implementations, stopping theetching process may be accomplished by removing the subassembly from theetchant, by either manual or automatic removal techniques.

Measuring the thickness of the subassembly and/or aluminum nitridesubstrate may be accomplished by various processes including mechanical,electrical, chemical, and/or optical techniques. For example, in someconfigurations, measuring the thickness of the AlN substrate may beaccomplished by optical techniques that use electromagnetic radiation.These techniques can involve a source that directs the electromagneticradiation toward the subassembly and a detector configured to detectelectromagnetic radiation that is diffracted, reflected and/ortransmitted by the subassembly. For example, the thickness of thesubassembly and/or AlN substrate may be measured using techniques suchas x-ray diffraction, ellipsometry, and/or interferometry. In someconfigurations, the measurement unit can arranged so that the thicknessmeasurements are accomplished in situ during the etching process. Inother configurations, the thickness measurements are made ex situ byremoving the subassembly from the etching process, making a measurement,and returning the subassembly to the etching process.

In some configurations, measuring the thickness of the AlN substrateinvolves detecting light that is transmitted through the subassembly.The wavelength range of the light used for such a measurement is in arange that is substantially absorbed by the bulk AlN substrate and istransmitted at least to some extent by the epitaxial layers. Asillustrated by the flow diagram of FIG. 3, when the etching process 320first begins, the AlN substrate is relatively thick and opaque to thelight. The light transmitted through the AlN substrate (if any) isdetected 330 during the etching process. As the AlN substrate is etchedaway, the AlN substrate becomes thin enough to allow transmission of thelight through the substrate. An amount of light transmission through theAlN substrate a change in light transmission compared to a baselineamount measured earlier in the etching process and/or a rate of changein light transmission through the AlN substrate can be detected 340 andmay be used to control 350 the etching process.

The etch rate of Al_(x)Ga_((1-x))N materials, where x=0 to 1, varies asa function of aluminum content. The graphs of FIGS. 4 and 5 illustrateetch times and etch rate, respectively, of epitaxially grown AlGaNlayers as a function of molar fraction of aluminum. For each sample, thethickness of the AlGaN layer being etched was monitored by periodicallytaking the sample out of the chemical bath etchant and performing x-rayscans to determine the x-ray signal intensity of the AlGaN layer. FIG. 4shows etch times of epitaxial Al_(x)Ga_((1-x))N layers having athickness of 2.4 μm etched in phosphoric acid (H₃PO₄) at a concentrationof 91% and temperature of 180° C. The layers of AlGaN having aluminummolar fraction of 74% or 67% were substantially removed in less than 5min., whereas the AlGaN layer having 47% aluminum mole fraction tookover 30 min. to remove.

FIG. 5 shows a graph of the etch rate of epitaxially grownAl_(x)Ga_((1-x))N in H₃PO₄ at the concentration of 91% and temperatureof 180° C. as a function of aluminum mole fraction. The graph shows thatthe etch rate increases with increasing aluminum molar fraction. FIG. 6is an example of the x-ray diffraction measurement of a samplesubassembly before 610 and after 620 removal of the substrate. Asdescribed in more detail below, in some embodiments, a layer ofrelatively high molar fraction epitaxially grown AlGaN can be used as aninterstitial layer to remove a bulk AlN substrate.

FIG. 7 is a block diagram of an apparatus that can be used to remove anAlN substrate from a subassembly by wet etching. The apparatus includesan etchant chamber 710 configured to contain the etchant and to hold thesubassembly being etched. The apparatus optionally includes ameasurement unit 720 configured to generate a signal indicative of thethickness of the AlN substrate and/or the thickness of the subassemblythat includes both the AlN substrate and the one or more epitaxiallayers. The signal is generated by the measurement unit 720 during theetching process and may reflect a series of thickness measurements. Thesignal may be used by a control unit 730 to control one or more aspectsof the etching process.

Control of the etching process may be accomplished by one or moresubunits of the control unit 730 that are arranged to control variousaspects of the etching process. For example, the subunits 731, 732, 733may control the temperature of the etchant, the concentration of theetchant, and/or the position of the sample. The etch rate of thesubassembly may be controlled by changing one or more of these and/orother parameters. The concentration control subunit 732 can beconfigured to maintain or change the concentration of the etchant byadding more water to the chamber 710 or by allowing the water in thechamber 710 to be reduced, e.g., by not replacing water that evaporatesduring boiling. The temperature control subunit 731 may include one orboth of heaters and coolers that heat and/or cool the etchant. Coolingthe etchant and/or decreasing the concentration of the etchant slows theetch rate. Increasing the temperature of the etchant and/or increasingthe concentration increases the etch rate up to a point. However, theetch rate can be dependent on hydrolysis of the AlN substrate surface.The etch rate may be limited when the concentration of the etchantbecomes too high and the amount of water in the etchant is insufficientto allow hydrolysis of the surface to occur. FIG. 8 illustrates the etchrate of bulk AlN as a function of temperature. As will be appreciated,the etch rate appears to level off for temperatures greater than about160 degrees C. which corresponds to a concentration of about 85%. Thisleveling off may be due to insufficient water to hydrolyze the surfaceof the substrate.

In some configurations, the temperature and/or concentration controlsubunits 731, 732 may be set up to maintain the temperature and/orconcentration of the etchant within predetermined ranges. Substrateremoval may include monitoring the etch solution. If the temperature ofthe phosphoric acid is maintained at the boiling point, loss of waterduring boiling reduces the rate of hydrolysis of the AlN and thusreduces the etch rate. The boiling point of phosphoric acid varies withthe concentration of the acid. For example, commercially availablephosphoric acid has an acid concentration of about 85% and boils atabout 158° C. As water is lost during boiling, the concentration of theacid and the boiling point increase. For a controlled etch rate theconcentration and/or temperature of the etchant may be maintained withinpredetermined ranges, e.g., concentration range of about 87% to about96% and a temperature range of 160° C. to 210° C. To maintain arelatively constant acid concentration, the lost water can be replaced,e.g., either with a reflux condenser or an automated drip system.

The measurement unit 720 includes one or more sensors or detectors(shown as sensors/detector s 722 in FIG. 7) coupled to suitableprocessing circuitry 721 configured to process signals from thesensors/detectors. During the etching process, the sensors/detectors 722detect changes in characteristics of the subassembly and/or the etchantthat are related to the thickness of the AlN substrate and/or thesubassembly. As previously mentioned, the measurement unit 720 can usevarious techniques to generate a signal indicative of thickness. Thesetechniques may comprise sensing optical, mechanical, chemical, orelectrical parameters of the AlN substrate, the subassembly and/or theetchant during the etching process. As an example, mechanicalmeasurement of the thickness of the subassembly may be made using aprofilometer. For this example, the profilometer probes are the sensorsof the measurement unit. The subassembly may be measured several timesduring the etching process. FIG. 9 shows superimposed graphs ofprofilometer measurements of the subassembly. Thickness Profile I is thestarting profile before the AlN substrate of the subassembly was etched.After the starting profile (Profile I) was acquired, the subassembly wasplaced in the etchant bath for a first period of time, then removed andthe thickness profile was remeasured. Profile A represents the thicknessof the subassembly after etching for the first period of time. AfterProfile A was acquired, the subassembly was etched for a second periodof time. After the second period of time, the thickness of thesubassembly was remeasured as shown in Profile B. The successiveProfiles I, A, B show the progress of the etching process after thefirst and second time periods.

Electrical characterization of the subassembly may be used to generatethe signal indicative of thickness. For example, in some embodiments,the sensors may comprise electrical contacts configured to makeelectrical contact to surfaces or portions of the subassembly. Theelectrical contacts are coupled to circuitry configured to implement oneor more electrical characterization processes. The electricalcharacterization of the subassembly may include one or more ofresistance/conductance measurements, capacitance measurements,current-voltage characterization, and or other characterizations. Adecrease in the thickness of the subassembly produces detectable changesin the electrical characterization of the subassembly.

The measurement unit may comprise chemical sensors and associatedcircuitry configured to detect chemical changes in the subassemblyand/or in the etchant. The chemical sensors can be positioned on thesubassembly, and/or in the etchant chamber, e.g., in the etchant bath.In some cases, the chemical sensors and associated circuitry areconfigured to measure a concentration of ions in the etchant bath. Thisconcentration may change during the etching process as a function of theamount of AlN that becomes dissolved in the etchant. Thus, ionconcentration as detected by the chemical sensors may be used togenerate the signal indicative of the thickness of the AlN substrate. Insome configurations, it may be helpful to position the chemical sensorsnear e.g., within a few mm, of the subassembly to increase thesensitivity to ion concentration at the surface being etched.

The measurement unit can use light transmitted through the subassemblyto generate a signal indicative of the thickness of the substrate. Inthese implementations, the measurement unit may include a light sourcearranged to direct light toward the subassembly and a detector arrangedto detect the transmitted light. The light source may comprise one ormore lamps, LEDs, and/or lasers emitting UV, visible, infrared orbroadband light. The light directed toward the subassembly includes awavelength range that is substantially optically absorbed by the AlNsubstrate and is not substantially absorbed by the epitaxial layers ofthe subassembly. For example, for a subassembly comprising a bulk AlNsubstrate and AlGaN epitaxial layers, the measurement unit may include alight source, e.g., LED, that emits light in a wavelength range fromabout 200 nm to about 365 nm and a detector that detects the light inthis range. In some configurations, the light source may emit and thedetector may detect light in a wavelength range of about 250 nm to about300 nm. In some cases, the light source is configured to output lighthaving a range of wavelengths and the detector is configured tocharacterize the spectral response of the subassembly over thewavelength range. Changes in the spectral response may be used tocontrol the etch process.

FIG. 10 is a block diagram of an apparatus 1000 that uses light togenerate the signal indicative of thickness. In the exampleconfiguration of FIG. 10, a subassembly 1010 including a bulk AlNsubstrate 1011 and one or more epitaxial layers 1012 on the AlNsubstrate 1011 is disposed in an etchant chamber 1020. Thermal elements1030, e.g., heaters and/or coolers, can be disposed in, on, or about thechamber 1020 for controlling the temperature of the etchant during theetching process. Additionally or alternatively, the apparatus 1000 mayinclude structures (not shown) configured to control the concentrationof the etchant in the chamber. A light source, e.g., a UV LED, isarranged so that light is directed towards the subassembly. Asillustrated in FIG. 10, the light may be directed towards thesubassembly 1011 through an optical fiber 1050. Prior to the etchingprocess, the light from the light source 1040 may be substantiallyblocked by the bulk AlN substrate 1011, but substantially transmitted bythe epitaxial layers 1012. As the substrate 1011 becomes thinner, thelight is able to be transmitted through the subassembly 1010. Thetransmitted light 1070 is detected by a photodetector 1080. In responseto the light 1070 transmitted through the subassembly 1010, thephotodetector 1080 generates an output signal 1090 indicative of thethickness of the AlN substrate 1011.

Another example of an apparatus 1100 that uses light to detect thethickness of the AlN substrate during the etching process is shown InFIG. 11. The apparatus 1100 in this embodiment includes an etchantchamber comprising a quartz tank 1120 which contains the phosphoric acidbath. The subassembly 1110 including an AlN substrate and one or moreepitaxial layers formed thereon is positioned and held in the tank 1120by the sample holder 1121. The apparatus 1100 includes two ports 1122,1123 each coupled to a vacuum. In this example, the ports 1122, 1123comprise closed ended evacuated quartz tubes. Light from the lightsource 1140 is guided by a first optical fiber 1150 through the firstport 1122 and is directed by a first mirror 1124 towards the subassembly1110. Light transmitted through the subassembly 1110 is directed by asecond mirror 1125 via a second optical fiber 1151 through the secondport. A photodetector 1180 is arranged to detect the light transmittedthrough the subassembly 1110. In response to the detected light, thephotodetector 1180 generates a signal 1190 indicative of the thicknessof the AlN substrate.

In some implementations, the epitaxial layers of the subassembly mayform a light emitting device such as an LED. In these implementations,the light emitting device may be the light source for the light that isused in generating the signal indicative of the thickness of the AlNsubstrate. FIG. 12 illustrates an apparatus 1200 that uses the epitaxiallayers 1212 of the subassembly 1210 to generate the light 1270 that isused to measure the AlN substrate thickness. The apparatus 1200 issimilar in some respects to FIG. 10, however the external light sourceand optical fiber shown in FIG. 10 are not used in the implementationillustrated in FIG. 12. The subassembly 1210 is disposed within theetchant chamber 1220 and includes layers 1212 which are epitaxiallygrown on the AlN substrate 1211. The epitaxial layers 1212 form a lightemitting device. The apparatus 1200 includes electrical contacts 1232,1233 that make contact to the light emitting device 1212 and leads 1234,1235 that connect the electrical contacts 1232, 1233 to a power supply1240. When the light emitting device 1212 is energized by the powersupply 1240 via the leads 1234, 1235 and electrical contacts 1232, 1233,the light emitting device 1212 produces light 1270 that is transmittedthrough the AlN substrate when the substrate becomes sufficiently thinto transmit the light 1270. As the AlN substrate 1211 is etched, ittransmits an increasing amount of light 1270 that is detected by thephotodetector 1280. The photodetector 1280 generates a signal 1290indicative of the thickness of the AlN substrate.

FIG. 13 shows an example of an idealized signal, e.g., generated by aphotodetector, that indicates the thickness of the AlN substrate.Initially, the AlN substrate is substantially optically absorbing to thelight transmitted toward the AlN substrate. When the etching processfirst begins, much of the light directed to the AlN substrate isabsorbed by the substrate and does not escape from the subassembly. Thethickness signal remains at a nominal value until the thickness of theAlN substrate becomes thin enough to transmit some light through the AlNlayer. When the aluminum nitride substrate becomes thin enough totransmit light, the thickness signal begins to increase as more and moreAlN is etched from the substrate. The change in the signal can be quiterapid when the thickness of the AlN substrate achieves a value thattransmits a substantial amount of light.

In some cases, the signal generated by the measurement unit can be usedto control the etching process. For example, the control unit maymonitor the signal until the point that a rapid change is detected. Atthat point, the control unit may operate to change the etch rate, e.g.,slow down or stop the etch process. The signal generated by themeasurement unit may be in used in conjunction with other information tocontrol the etching process. For example, in addition to the signalgenerated by the measurement unit, the control unit may take intoaccount factors such as the duration of the etching process, theconcentration of the etchant in the chamber over the duration of theetching process, and/or the temperature of the etchant over the durationof the etching process. The control unit can control the operation ofthe thermal units, e.g. heaters or coolers, and/or can control theconcentration of the etchant to change the rate of the etching process.

Various types of light emitting devices may be formed using theprocesses for bulk AlN substrate removal described herein. Removal orthinning of the bulk AlN substrate is particularly useful for UV LEDsand UV laser diodes that emit light in the wavelength range of about 200nm to about 365 nm or about 250 nm to about 320 nm. Although the use ofbulk AlN substrates in the formation of these light emitting devicesallows the growth of low defect epitaxial layers in the devicestructures, the AlN substrate substantially absorbs light emitted fromthe active region towards the substrate side of the device. Thus,removal of the substrate allows increased light emission from thesubstrate side (back side) of the device. In some devices, the upperlayers of the light emitting device on the opposite side of the activelayer from the substrate (i.e., layers on the front side of the device)may be optically absorbing at the wavelength of light generated by thedevice. Thus, it is helpful if light generated by the device can beemitted through the substrate side which is made possible by removal ofthe AlN substrate.

FIG. 14 shows an embodiment of an UV LED 1400 that is initially grown ona bulk AlN substrate 1410. The growth of the epitaxial layers may beaccomplished, for example, using metal organic chemical vapor deposition(MOCVD), by hydride vapor phase epitaxy (HYPE) and/or by any combinationof these techniques. The bulk AlN substrate 1410 may be subsequentlyremoved or thinned as indicated by arrow 1411. In some cases, the AlNsubstrate 1410 may be removed. In some cases, the AlN substrate 1410 maybe thinned, e.g., thinned beyond what is possible using mechanicaltechniques, to a thickness of about 50 μm or less. The UV LEDheterostructure 1400 illustrated in FIG. 14 may be grown by metalorganic vapor epitaxy to include the following:

an n-side Al_(x)Ga_(1-x)N transition region 1420;

an n-doped Al_(0.74)Ga_(0.26)N n-contact layer 1430;

an n-side Al_(0.74)Ga_(0.26)N separate confinement heterostructure (SCH)1440;

a multiple quantum well (MQW) light emitting layer 1450, e.g., designedto emit at a central frequency of λ=250 nm;

an electron blocking layer 1460;

a p-side AlGaN SCH 1470, a p-doped AlGaN transition region 1480; and

a p-doped GaN contact layer 1490.

The n-side Al_(x)Ga_(1-x)N transition region 1420 includes an initialsurface of epitaxial growth 1405 formed on the AlN substrate 1410. Insome configurations, the n-side transition region 1420 is a series ofshort period superlattices with average Al compositions ranging from xequal to about 0.90 to x equal to about 0.80. In some configurations,the transition region 1420 may be a linearly graded alloy with x rangingfrom about 100% to about 74%. The n-contact layer 1430 is the contactlayer for the n-contact 1435 and may be about 1.9 μm thick, for example.The MQW light emitting layer 1450 can comprise 3 pairs of 6 nm-thickAl_(0.68)Ga_(0.32)N barriers and 2 nm-thick Al_(0.65)Ga_(0.35)N wells.The electron blocking layer (EBL) 1460 may be about 83% aluminum andabout 20 nm thick. The p-SCH 1470 can have an aluminum composition, x,ranging from about 0.3 to about 0.74. The p-transition layer 1480 can bea superlattice or can be an alloy with x ranging from about 0.3 to about0.74. The p-contact layer 1490 comprises GaN and is a contact layer forthe p-metal contact 1495. The described structure is only an example.The number of layers, composition of layers, the actual heterostructuredesign, and/or the growth procedure can vary significantly. Inparticular, the layers can contain indium to form InGaN or InAlGaNinstead of AlGaN.

The LED is initially grown on a bulk AlN substrate and the substrate issubsequently removed or thinned. This process produces a defect densityat the initial surface of epitaxial growth in the AlGaN transition layerto be relatively low, e.g., in the range of about 10¹⁰ cm⁻³ to about 10³cm⁻³ such as about 10⁸ cm⁻³ or 10⁷ cm⁻³. Thus, the device retains thehigh-quality epitaxial growth afforded by substantial lattice matchingto the AlN substrate but can also allow substantial light transmissionthrough the substrate side of the device.

For example, an AlGaN n-transition layer having Al molar fraction ofless about 70% to 80% that is epitaxially grown on the bulk AlNsubstrate would be closely lattice matched to the substrate. In thisscenario, the AlGaN n-transition layer is initially strained to achievea close match to lattice constant of the bulk crystalline AlN with lowdefect density. When the AlN substrate is removed, the lattice of then-transition layer relaxes, but the low defect density is retained.

FIG. 15 illustrates an LED that is similar in some respects to the LED1400 of FIG. 14. The elements identified by reference numbers in FIG. 15are similar to their counterparts in FIG. 14. The LED 1500 differs fromthe LED 1400 in FIG. 14 because it includes an etch reduction layer 1510(also referred to as an etch stop layer) that may be grown between theAlN substrate 1410 and the AlGaN transition layer 1420 prior to theetching. In this example, the etch reduction layer 1510 includes theinitial epitaxial surface 1505. The etch reduction layer 1510 may beimpervious to the etchant used to etch the AlN substrate, or the etchreduction layer's etch rate in the etchant may be lower than the etchrate of the AlN 1410. The etch reduction layer 1510 may be a relativelythin GaN layer or an AlGaN layer having an Al content (Al molarfraction) in a range of about 70% to about 80% and/or may comprise othersuitable materials that provide etch reduction capability, for example.

Substrate removal by, either complete removal or chemically etching toleave a substrate remnant having a thickness less than about 50 μmenables access to the backside of a device for subsequent processing.The backside an amount of surface roughness or texturing is present onthe backside of the device after the substrate is removed. In somecases, the texturing may be imparted to the backside of the device dueto the substrate removal process. The substrate removal process may beconfigured to provide texturing of the back side of the device duringthe substrate removal process The surface roughness resulting from thechemical etch depends on process parameters such as etch bathtemperature, acid concentration, etch rate, and/or wafer backsidepolish. Appropriate process parameters can be chosen to produce desiredsurface roughness characteristics. In some the backside of the devicecan be textured after substrate removal, e.g., by ion beam or chemicalprocessing. Texturing can enhance light extraction. In some scenarios,without backside texturing, a substantial amount of emitted light can betrapped in the heterostructure through total internal reflection.Backside texturing scatters the emitted light, frustrates total internalreflection, and enhances light extraction efficiency.

Surface texturing can be patterned by various wet or dry processingtechniques. For example, the surface can be textured by immersion inpotassium hydride (KOH) solutions. Plasma processing techniques can alsobe used to roughen the surface. For example the initial epitaxialsurfaces 1405, 1505 of LEDs 1400, 1500, may be textured surfaces.

Back side texturing is particularly important in short wavelength UVLEDs because the emitted light becomes increasingly polarized in the TMdirection (perpendicular to the surface plane) at shorter wavelengths.Consequently, most of the light is emitted propagating toward the deviceedges, rather than toward the back side surface. With the substrateremoved, features can be patterned to redirect the emitted light towardand through the back side.

Substrate removal also enables vertical electrical injection LEDarchitectures. FIG. 16 shows an illustration of such a structure 1600.The LED includes an n-type heterostructure 1630 that includes theinitial epitaxial surface 1631. The LED 1600 was grown on a bulkcrystalline AlN substrate (not shown in FIG. 16) starting with theinitial epitaxial surface of the n-type heterostructure and continuingthe epitaxial grown with the light emitting layer 1620 and p-typeheterostructure 1610. After formation of the epitaxial layers 1610-1630,the bulk AlN substrate is removed.

P-contacts 1605 of the vertical injection LED 1600 are soldered 1603 topads 1602 of a heat sink chip 1601. The electrodes 1640 are electricallycoupled to the heat sink chip 1601 through one or more leads 1641 andn-contacts 1642. During operation of the device, electrical current isinjected from an external supply (not shown) through the p-contacts1605, through the p-side (p-type heterostructure 1610) through the lightemitting layer 1620, through the n-side (n-type heterostructure 1630 andis collected at electrodes 1640 formed on the backside of the device1600. Removing the electrically insulating AlN substrate allows accessto the n-heterostructure layer 1630 for forming the backside contacts1640.

The vertical injection structure 1600 illustrated in FIG. 16 can beespecially beneficial when the material that forms n-typeheterostructure 1630, e.g., n-doped AlGaN, is very resistive. In thissituation, without removal of the AlN substrate, electrical currentwould be collected at an area beside the light-emitting area 1620, onthe same side of the device 1600 as the p-contacts 1605 (for example,see the structures of LEDs 1400, 1500 1600 of FIGS. 14 and 15). Toobtain higher light output powers, the area of light emitting layer 1620is made to be relatively large. Electrical current injected at thecenter of the light emitting layer would have to travel a longer lateraldistance within the resistive n-doped layer before reaching anre-contact at the periphery of the device. This current path over thelateral distance could produce large voltage drops and could causenon-uniform current injection. Removal of the AlN substrate allowscontacts to be formed at the back side of the device, thereby providinga more direct current path and less voltage drop between the p-contacts1602 and the current collection electrodes 1640.

Laser diodes (LD) can also benefit from AlN substrate removal orthinning FIG. 17 shows an energy barrier diagram of typical laser diodedesign (solid line) along with the associated optical mode profile(dashed line). The central frequency of the mode corresponds to awavelength of about 250 nm. In this design, the overlap between theoptical mode and the quantum wells 1730 is represented by Γ=8.26%. TheLD includes an n-contact 1710, n-SCH 1720, multiple quantum wells 1730,p-SCH 1740, and p-contact 1760.

In a typical UV laser diode made on AlN substrates, substantial modalloss would result if any electric field penetrates into the AlNsubstrate, because the AlN substrate is highly optically absorbing atthe lasing wavelength. An optically absorbing substrate contributes towaveguide materials loss, thus increasing the required gain to achievelasing. The substrate-side cladding layer needs to be sufficiently thickin order to tightly confine the optical mode within the active region. Athick n-cladding layer, however, contributes to device resistance.Removing or thinning the AlN substrate reduces modal loss and addsdesign flexibility. Removing/thinning the AlN substrate also enablesvertical injection LD structures that provide benefits analogous to theones described in the LED case.

In some embodiments, substrate removal/thinning may be accomplished onlyin certain sections of the back side rather than across the entire backside surface. The substrate can be protected from the chemical etchant,and openings at select areas can be made to etch only those areas. Forexample, openings can be made only at regions directly below thelight-emitting layers, so portions of the bulk substrate remain afterthe etching process and are still available for handling.

In some implementations, epitaxially grown AlN or AlGaN templates and/orsubstrates, referred to collectively herein as templates/substrates, canbe produced as stand-alone components. These templates/substrates can besold as piece parts, for example, and subsequently used for formation ofnumber of types of devices, including optoelectronic devices such asLEDs and laser diodes and electronic devices such as heterojunctiontransistors and amplifiers. The epitaxial templates can be formed bygrowing epitaxial AlN or AlGaN on a bulk crystalline AlN substrate andthen removing or thinning the AlN substrate. This technique can producehigh quality, optically transparent epitaxially-growntemplates/substrates of nitride system materials, such as AlN, AlGaN,InN, InGaN, and/or InAlGaN, which can be used as a base for epitaxialgrowth in subsequent processing steps. The epitaxial template/substratemay be grown at relatively high growth rates, e.g., above about 1 um perhour to to produce a layer of about 30 μm to about 400 μm. Theepitaxially-grown templates/substrates may be formed by metal organicchemical vapor deposition (MOCVD), by hydride vapor phase epitaxy (HVPE)and/or by any combination of these techniques. High quality, opticallytransparent templates/substrates may be useful for UV optoelectronicdevices for the reasons described earlier. The most common orientationis c-plane AlN, but other crystal orientations such as m-plane, a-plane,or semi-polar orientations can also be obtained by cutting the AlN bulksubstrate in the desired orientation prior to initiating the epitaxialgrowth.

FIGS. 18 and 19 illustrate the process (FIG. 19) of forming astand-alone epitaxially grown template/substrate and the resultingstructure (FIG. 18). An epitaxial layer, e.g., comprising epitaxial AlN,AlGaN, InN, InGaN, and/or InAlGaN, is grown on a bulk crystalline AlNsubstrate 1910. The resulting structure 1810 is illustrated in FIG. 18.The bulk AlN substrate is removed or thinned 1920 by processespreviously described herein, leaving the stand alone epitaxialtemplate/substrate 1820.

In some implementations, a structure, e.g., an epitaxialtemplate/substrate, optoelectronic device, or other structure orheterostructure can be grown on an interstitial layer that is grown onvarious substrates such as AlN, silicon, sapphire, or SiC. These can beseparated from the substrate by etching the interstitial layer, thusseparating the structure from the substrate. FIG. 21 illustrates theprocess of forming a stand-alone, epitaxially grown heterostructure fromits substrate by etching the interstitial layer. FIG. 20 shows theinitial and final states of the structure. An interstitial layer 2020 isepitaxially grown 2110 on a substrate 2010. The structure 2030 is grown2120 on the interface layer 2020. The interface layer 2020 is laterallyetched 2130, separating the structure 2030 from the substrate 2010.

In the foregoing detailed description, numeric values and ranges areprovided for various aspects of the implementations described. Thesevalues and ranges are to be treated as examples only, and are notintended to limit the scope of the claims. For example, embodimentsdescribed in this disclosure can be practiced throughout the disclosednumerical ranges. In addition, a number of materials are identified assuitable for various facets of the implementations. These materials areto be treated as exemplary, and are not intended to limit the scope ofthe claims. The foregoing description of various embodiments has beenpresented for the purposes of illustration and description and notlimitation. The embodiments disclosed are not intended to be exhaustiveor to limit the possible implementations to the embodiments disclosed.Many modifications and variations are possible in light of the aboveteaching.

The invention claimed:
 1. An apparatus, comprising: a chamber configuredto expose a bulk aluminum nitride (AlN) substrate to an etchant duringan etching process, the bulk AlN substrate being a substrate of asubassembly comprising one or more epitaxial layers grown on the bulkAlN substrate; and a measurement unit configured to generate a signalindicative of a thickness of the bulk AlN substrate and a light sourceconfigured to generate output light at a wavelength that issubstantially absorbed by the bulk AlN substrate and is notsubstantially absorbed by the one or more epitaxial layers, themeasurement unit additionally comprising a detector arranged to detectthe light transmitted through the subassembly, the detector beingconfigured to characterize the spectral response of the subassembly overa wavelength range of the detected light.
 2. The apparatus of claim 1,wherein the measurement unit includes contacts configured to makeelectrical contact with the subassembly and the measurement unit isconfigured to measure one or more electrical characteristics of thesubassembly through the electrical contacts.
 3. The apparatus of claim1, wherein the measurement unit includes a chemical sensor configured tosense an ion concentration in the chamber near the subassembly and themeasurement unit is configured to generate the signal based on the ionconcentration.
 4. The apparatus of claim 1, wherein the signal is ananalog signal that indicates the thickness of the aluminum nitridesubstrate over a thickness range.
 5. The apparatus of claim 1, whereinthe signal indicates that the AN substrate has been substantiallyremoved in a region of the subassembly.
 6. The apparatus of claim 1wherein the etchant comprises phosphoric acid at an elevatedtemperature.
 7. The apparatus of claim 6, wherein the phosphoric acidhas a concentration in a range of about 87% to about 96% and having atemperature in a range of about 160° C. to 210° C.
 8. The apparatus ofclaim 1, wherein the light source emits light in a wavelength range fromabout 200 to about 365 nm.
 9. An apparatus, comprising: a chamberconfigured to expose a bulk aluminum nitride (AlN) substrate to anetchant during an etching process, the bulk AlN substrate being asubstrate of a subassembly comprising one or more epitaxial layers grownon the bulk AlN substrate; and a measurement unit configured to generatea signal indicative of a thickness of the bulk AlN substrate andcomprising a detector arranged to detect light transmitted through thesubassembly, wherein the light is generated by the subassembly andtransmitted to the detector when the bulk AlN substrate becomessufficiently thin.
 10. The apparatus of claim 9, further comprising acontrol unit configured to control the etching process based on thesignal.
 11. The apparatus of claim 10, wherein the control unit isconfigured to reduce the etch rate when a change in the signal exceeds apredetermined limit.
 12. The apparatus of claim 10, wherein the controlunit is configured to stop the etching process when a change in thesignal exceeds a predetermined limit.
 13. The apparatus of claim 10,wherein the control unit is arranged to provide thermal energy to theetchant in the chamber.
 14. The apparatus of claim 13, wherein thecontrol unit comprises one or both of a heater configured to heat theetchant and a cooler configured to cool the etchant.
 15. The apparatusof claim 10, wherein the control unit is configured to control aconcentration of the etchant.